A typical example of a known semiconductor integrated circuit having a built-in capacitor is illustrated in FIGS. 1 and 2. The semiconductor integrated circuit herein shown includes a semiconductor substrate 10 having a relatively thick insulating layer 12 formed on the surface thereof. On the surface of the insulating layer 12 in turn is formed a polysilicon conductive layer 14 which is in part covered with a relatively thin polysilicon oxide film 16. The polysilicon oxide film 16 in turn partly underlies a polysilicon conductive layer 18. The lower polysilicon conductive layer 14 and the upper polysilicon conductive layer 18 form part of a built-in capacitor 20 which has a dielectric layer formed by the thin polysilicon oxide film 16 intervening between the electrode plates formed by the two conductive layers 14 and 18. The exposed portions of the polysilicon oxide film 16 and the upper polysilicon conductive layer 18 are covered with an insulating layer 22 which is formed with contact holes or openings 24 and 26. One contact hole 24 extends to the surface of the lower polysilicon conductive layer 14 and the other contact hole or opening 26 extends to the surface of the upper polysilicon conductive layer 18. The contact hole 24 and opening 26 are formed typically by photoetching techniques using an etchant of hydrofluoric acid. On the upper polysilicon conductive layer 18 and the toplevel insulating layer 22 are deposited wiring metal layers which include a metal layer 28 contacting the surface of the lower polysilicon conductive layer 14 through the contact opening 26 and a metal layer 30 overlying the surface of the upper polysilicon conductive layer 18 through the opening 26 in the insulating layer 22. Each of these wiring metal layers 28 and 30 is typically of aluminum. The capacitor 20 thus formed in the semiconductor integrated circuit has its lower polysilicon conductive layer 14 isolated from the semiconductor substrate 10 by means of the thick insulating layer 12 and for this reason provides a sufficiently small capacitance which makes the capacitor less susceptible to the potential on the substrate 10.
The semiconductor integrated circuit having such a capacitor 20 formed therein however has a drawback in that the contact opening 26 to receive the wiring metal layer 30 for the upper polysilicon conductive layer 18 is so spacious that contact spiking tends to take place between the polysilicon conductive layer 18 and the metal layer 30. During a high-temperature process step which is used to improve the ohmic contact between the conductive layer 18 and the metal layer 30 which is usually of aluminum, a large quantity of aluminum is caused to sink into the underlying polysilicon conductive layer 18 and impedes formation of proper ohmic contact between the layers 18 and 30. The aluminum spike may even penetrates through the polysilicon conductive layer 18 into the polysilicon oxide film 16 and may thus damage the oxide film 16.
Another problem results from the use of an etchant of hydrofluoric acid during etching of the insulating layer 22 for the formation of the contact hole and opening 24 and 26. Not only the insulating layer 22 but also the upper polysilicon conductive layer 18 are subjected to the attack of the etchant of hydrofluoric acid during this etching step. A small quantity of hydrofluoric acid is thus allowed into the conductive layer 18 through the boundaries between the polysilicon crystals forming the layer 18 and may reach the underlying polysilicon oxide film 16. This, again, may cause damage to the polysilicon oxide film 18. The larger the area of the contact opening 26 in the insulating layer 22, the more serious the damage thus caused to the oxide film 18 by the attack of hydrofluoric acid will be. It is however desirable that the contact opening 26 in the insulating layer 22 be as large in area as possible to reduce the contact resistance and provide uniformity of field between the upper polysilicon conductive layer 18 and the metal layer 30 thereon.
The present invention contemplates elimination of these problems which have thus far been inherent in semiconductor integrated circuits incorporating built-in capacitor devices.